This application claims the priority benefit of Taiwan application serial no. 90120462, filed Aug. 21, 2001.
1. Field of Invention
The present invention relates to a bonded anisotropic conductive film. More particularly, the present invention relates to a bonded anisotropic conductive film for a flip chip package.
2. Description of Related Art
In this information explosion age, the market for multimedia is expanding at an ever-increasing rate. Integrated circuit packaging has to follow the market trends demanding for more digital equipment, networking, local area connections and customization. To meet such developmental trends, electronic devices must have high-processing, multi-functional, integrated capacity. Moreover, the devices must be light, compact and have a moderate to low price. Hence, miniaturization and high-level integration are the major driving forces behind the development of a new generation of integrated circuit packages. Ball grid array (BGA), chip-scale package (CSP), flip chip (F/C) and multi-chip module (MCM) are some of the high-density integrated circuit packages currently manufactured. Among these packages, flip chip occupies a relatively small area and accommodates a large number of leads. Inside flip chip, the lead wires are short, inductance is low and noise control is easy. Hence, most flip chips are used in high-end package products.
FIG. 1 is a schematic diagram showing a conventional method of using an anisotropic conductive film to electrically connect a silicon chip with a carrier. In most flip chips, input/output (I/O) contact points 102 on a silicon chip 100 and contact points 106 on a carrier 104 are electrically connected using an anisotropic conductive film (ACF) 108 as a medium. The anisotropic conductive film 108 is fabricated using a plastic material 109 and conductive particles 110. In general, the conductive particles 110 are formed using such material as gold (Au). Hence, when a pressure is applied to the silicon chip 100 and the carrier 104, the conductive particles 110 within the anisotropic conductive film 108 form an electrical connection linking the contact point 102 and the contact point 106. Since the conductive particles 110 within the anisotropic conductive film 108 are the medium for connecting up the contact points 102 on the silicon chip 100 with the contact points 106 on the carrier 104, resistance is usually high and reliability of the connection is usually low.
FIG. 2 is a cross-sectional view of a typical conductive particle inside a conventional anisotropic conductive film. As shown in FIG. 2, the structure of a conductive particle 110 inside an anisotropic conductive film 108 comprises a near spherical plastic bead 112 and a gold plated layer 114 over the plastic bead surface.
When an anisotropic conductive film is used as a medium for connecting contact points on the silicon chip 100 with contact points 106 on the carrier 104, pitch of the contact point 102 cannot be too small. This is because the gold plated layer 114 of the conductive particle 110 may lead to a short circuit between neighboring contact points. In general, pitch P between contacts 102 must be greater than five times the diameter D of the contact 102 to prevent any unexpected short circuit due to contact with a neighboring contact point. Hence, constrained by the intrinsic structure of the conductive particles within the anisotropic conductive film 108 (the gold coating 114 over the plastic bead 112), further reduction of pitch P between neighboring contact points 102 is restricted.
In brief, using a conventional anisotropic conductive film having conductive particles therein, reliability of electrical connection is low and resistance after connection is high.
Accordingly, one object of the present invention is to provide a bonded anisotropic conductive film capable of forming a conductive pathway between a silicon chip and a carrier through bonding such that electrical connectivity of the conductive path is more reliable and resistance of the conductive path is lower.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a bonded anisotropic conductive film. The conductive film is fabricated using a plastic material and a plurality of conductive particles inside the plastic material. The plastic material, for example, is a thermal set material hardening at a first temperature. Each conductive particle inside the plastic material includes a conductive bead, a bonding layer and a flux layer. The bonding layer is formed over the conductive bead enclosing the conductive bead entirely. The flux layer is formed over the bonding layer. The bonding layer has a melting point at a second temperature where the second temperature is higher than the first temperature.
The conductive particles inside the bonded anisotropic conductive layer of this invention have an electrically insulating outermost flux layer. The flux layer not only serves as an insulating layer, but also brings the minimum separation between contact points closer together. Furthermore, the flux layer also has the capacity to cleanup any impurities on the contact surface of the silicon chip or the carrier. Hence, the bonded anisotropic conductive film forms a better common metallic bonding with the contact points.
The bonded anisotropic conductive layer of this invention can be applied to the fabrication of a flip chip. A silicon chip, a carrier and a bonded anisotropic conductive film are provided. The bonded anisotropic conductive film is placed between the silicon chip and the carrier and a pressure is applied so that contact points on the silicon chip and contact points on the carrier contain some conductive particles. In the meantime, the bonded anisotropic conductive film is heated to a first temperature, after heating the plastic material within the bonded anisotropic conductive film is hardened. Thereafter, the bonded anisotropic conductive film is heated to a second temperature so that the regions between the contact points on the silicon chip and the contact points on the carrier contain conductive particles. Through these conductive particles, the silicon chip and the carrier are electrically connected via their respective contact points and common metallic bonds.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.